A typical dispensing device for supplying liquid, such as hot melt adhesive, generally includes a body having a valve stem that opens and closes a dispensing orifice. The valve stem is usually actuated in at least one direction by pressurized air to dispense discrete amounts of pressurized liquid. Either a spring mechanism or pressurized air is used to move the valve stem in an opposite direction against a valve seat. This stops the flow of liquid from the dispensing orifice.
More specifically, devices generally related to the present invention include a liquid passage adjacent the dispensing orifice and an actuator cavity or chamber at an opposite end of the device. The actuator cavity contains a portion of the valve stem which is connected with a piston member and which is also connected with a spring return mechanism, as discussed above. Under sufficient air pressure applied on one side of the piston member, the valve stem is moved in a direction away from the valve seat to discharge liquid. When the air pressure is relieved, the spring mechanism will automatically return the valve stem to a normally closed position against the valve seat. Such spring mechanisms generally include an adjustment to vary the spring compression and thereby vary the amount of air pressure required to open the valve. Adjustment of the spring compression will also adjust the biasing force used to close the valve. These devices also include a stroke adjustment, or the spring compression adjustment also varies the stroke of the valve stem to adjust the flow rate.
Despite the wide success of devices as described above, improvement is desired. For example, a dynamic seal placed generally between the dispenser body and the moving valve stem typically prevents liquid from leaking into the actuator cavity. Dynamic seals are conventionally understood to be seals between two surfaces that move relative to one another. These dynamic seals may press tightly against the valve stem and cause friction and seal wear. The higher friction may place greater demands on the requirements for pressurized air to move the valve stem. On the other hand, selecting a looser dynamic seal could result in inadequate sealing, thus allowing the liquid to bind the piston and pressurized air to enter into the liquid passage, causing undesired dispensing discontinuities. Even with reduced friction, the dynamic seal will wear over time and lose its ability to seal properly.
It would therefore be desirable to provide a dispenser that eliminates or reduces the need for dynamic seals in contact with the pressurized liquid, thus eliminating or reducing problems such as those mentioned above.